Hey luchin, I'm very happy that "vibin champ" proved so useful to you! 8|
I also searched for a "fender" sounding fet circuit a lot and this seemed very simple/useful  :tu:

I have yet to build it (still haven't found the time), but my plans are similar:
I will use a "mid" pot (around 20 to 30k) and some lower gain transistors (MPF102/2N5457).

Good luck, and use it well! 

Hi luchin!
Yeah, a lot of us tried to do some project like this after reading about and building a Fetzer Valve... ROG are great! :tu:
Well, while I like fets more than opamps for dirty tones, I never tried an opamp for a buffer (and I don't need to, as I have a LOT of fets available). The type of buffer shouldn't make much difference noise-wise. You say that post tone stack gain adds a lot of noise? What kind of power supply are you using? A cheap power supply can bring all sorts of nasty things into your sound... xP ...but if you are using a battery, the problem is probably elsewere. (do you have a schematic?)

I've also been looking at THIS "fender-ish" circuit. Something to try and fiddle with, when I have time

At the moment I'm spending most of the time tinkering with my dirty metal preamp (dr.boogey & mods). When I'm satisfied with it, I will return to this clean preamp with fender tone stack. After that I will also make a marshall(ish) preamp and integrate all three into a "head" powered by a LM3886 or something. That will have to be more than enough sounds for my home/practice amp 

Hello again after some time! 
I built the preamp and it works great  :tu:
I used mensur's input resistor (4k7), though I still don't know why is it necessary... 

The next thing I will try to do is to include a simple fender-like "tone stack", a.k.a. bass/mids/treble eq.

The generic schematic looks like this:


So if I understand correctly, I should place the tone stack input directly at the source of the buffer.
I suppose the 1uF "output capacitor" can be easily omitted, because the three capacitors inside the tonestack (C1, C2, C3) already block DC voltage?

And then again, if I need low impedance output, I can just place another gain stage + buffer after the tone stack, right? This would also serve as "recovery" for the signal loss of the tone stack. As for volume control - I think 1M voltage divider between the tonestack and the second gain stage is pretty common. If I were to use an J201 transistor (very low Vp) for the second stage, I think I could even get some "dirt" out of it


I will try to play with these ideas some more and post my results... hopefully with some photos 

He he, I just finished drawing my layout and there you go, done it before me! 
I have no experience with drawing a layout myself, so I'll trust your comments about this. So far I only etched pcb's that were drawn by others. I thought this could be my first 

What I did is kind of a more compact version.
I first drew everything together with jacks and pot wiring:


I tried to keep the path (traces) of guitar signal as short as I could. Also, a while back I did a few mini amps and preamps from runoffgroove.coma with this kind of "star" grounding and it worked realy good (and dead silent), so I thought of using it here as well. All grounds (pcb, in_jack, out_jack, pot) will be joined at the power supply (0V) with bolt/nut. And just in case, input, output and pcb-to-pot wires will be shielded. (helped me a lot with high gain pedals)

Then I removed all the components and flooded the empty layout space with the ground plane.

I am aware this can create a few benefits (and possibly problems), but I'm not experienced enough to predict what can go wrong. Do you maybe see a problem with this?

And just now I noticed a new resistor at the input in your diagram: 4.7k... what does it do? 
I can easily add it in my layout, there is enough space in the input area.

I also used a power filtering cap, but 100uF instead of your 1000uF.
There is still enough space for the bigger one. I guess the bigger the better (filtering).

No no, I did read it well... 
I just wanted to confirm the same results by calculating it again myself.  :tu:
(active learning as opposed to passive)

Well, thank you very much (puno ti hvala!! 8|), you've helped me gain a lot of new knowledge. :tu:

Now that calculations are behind me, I can finally draw the new schematic:


The final thing I now must do, is to draw and etch a PCB :tu:  (I'm tired of perfboard)


Well, while I'm at it...


I have an enclosure measuring L/H/D = 12 x 4 x 18 cm.
I would like to mount the preamp in this enclosure, together with the power supply.
Power supply would be a simple one, small 15V trafo, around 1000uF smoothing, LM7818 regulator.

I think this would be ok:


Would the close proximity of transformer create any serious noise in the preamp?
Distance between the transformer and preamp PCB would be around 10cm 

I could always use a separate power supply, but I would like to have a single practical unit, that works by itself 

I was a kid and bit too young to remember anything from the old Yugoslavia, but they say that were the good times 

Anyway, thank you both. I went through some online theory pages and found a few other equations that helped me understand the few missing relations. I think I get it now... I hope :duh


I will now go again through the calculations for my transistor:

These are the equations I used for the gain stage:

Vgs = -Id*Rs
Vds = Vdd-Id*(Rd+Rs)
Id = Idss*(1-(Vgs/Vp))^2 (the characteristic curve is drawn from this equation)
gm0 = 2*IDSS/VP
gm = gm0*(1-(VGS/VP))
Av = (gm*Rd)/(1+gm*Rs) (if there is no Cs)

My transistor:
MPF102
Vp = -4.5V
Idss = 13.5mA

I choose Vgs to be 1/2 of Vp, so
Vgs: 0.5*(-4.5V) = -2.25V

Id = 13.5mA*(1-(2.25V/4.5V))^2 = 13.5mA * 1/4 = 3.375mA
Rs = Vgs/Id = 2.25V/3.375mA = 666ohm (closest is 680R)

gm0 = 2*13.5mA/4.5V = 0.006mS
gm = 0.006mS*(1-(2.25V/4.5V)) = 0.003mS

Then I choose Rd. I understand that with larger Rd come larger voltage gain and output impedance + lower current.

Rd = 2.2k  (18V/2200R = 0.0081A = 8mA)
Av = (0.003mS*2200R)/(1+0.003mS*680R) = 2.17 (= 6.7dB)


Ok, I hope everything is OK this far.

Then comes the buffer (aka "source follower" or "common drain amplifier") 
Only the Rs is necessary to bias it. I've read some theory about it, but I'm not sure. The way I understood it, you have to choose Rs in a way, that at the source you have half the voltage (Vcc). If this is correct, I then used the identical MPF102 transistor with calculated Id = 3.375mA (for Vgs = 1/2 Vp) inserted in the equation:

1/2 Vcc = 9V = 3.357mA * Rs
Rs = 9V / 0.003357A = 2680R = 2.7k

Is this ok, or am I making things up?  :loco 

He he, that IS a very short answer 

So you ment with 18V supply I should use Rs:15k and Rd::22k?
(I didn't fully understand that sentance, sorry)

For which transistor would this be?

About that capacitor: as far I understand the Cs (in parallel with Rs) has an effect on increasing gain... and also acts as some kind of frequency filter (which I don't understand). If I can get around 2x or 3x gain without it, as you recommended, that is probably fine by me.

BC546 is something easily available, so I will take it into account. Using bipolar NPN transistor would further lower the output impedance, right? Some other benefits besides that?

About that output cap: I have a couple 1uF/63V polyester foil caps, that I can use. Not realy WIMA, but no doubt better than any electrolytic I think. My soundcard is nothing special (M-Audio 1010lt) and realy, I doubt I would hear the difference.

Quote from: J M Fahey on September 25, 2010, 12:30:12 PM
Of course Mensur's calculations are correct, but I fear you find this complicated and give up, unnecessarily.

I do indeed, but I am also curious and like to learn new (complicated) things... so I won't give up that easily  :tu: I feel these little projects are all about learning new stuff, otherways I would have bought a cheap (active) DI box or something similar


Hehe, English is a language but Ciryllic is a writing... (which I can't read/write)
Though, we could communicate just fine in some of our languages of the Balkans, but I doubt anyone else would be interested in reading that 
This way anyone can learn from this posts :tu:

Great, so this corresponds to what I already knew: gain is almost unity and output impedance around few hundred ohms (for buffer).

I went and measured a few of my 2N5457 and MPF102 transistors.
Most of 2N5457 were around Vp:-1.5V, Idss:2.5mA
and MPF102 were quite different: Vp:-4.5V, Idss:13.5mA

So I suspect MPF102 is better for my application (more current)?
One problem though, I couldn't find any graph of Id/Vgs relation for it (datasheet doesn't have it).

Is there a way to calculate/draw it myself?
I found this equation on wiki (for linear region):

...but I dont know what is Vds. I suspect this is hard to do without measuring..?

I need to have something like this, right?
(example for Vp:-4V and Idss:14mA)


Is it ok to just approximate? 

Wow, you've given me a lot of theoretical knowledge. I now have to slowly digest it :tu:

I think now I understand the DC coupling effect. And for your example - that is VERY helpful. I always felt that one could learn a lot from this kind of examples, even without serious theoretical background.

Ok, so if I understand your calculations are for the "gain stage" of this preamp? (replacement for "fetzer valve"?)
I went through everything again and learned how to modify parameters for other transistors, using Idss, Vp and other relations from datasheet. This should not be a problem any more.

The only thing left that I don't understand is, how to determine the Rs of the buffer. The most common values I found in other designs were between 3.3k and 10k. Can this be calculated to something more optimal? How to do it? What should be the Vgs (or Vrs)?

Thanks again for all the help! 

Ok, the voltage divider is gone.



I understand if the audio signal passes through a cap, it has to be a good one.
For that 1uF I will pick something nice, polyester or possibly polypropilene.
It doesn't really have to be polarized, right? (like in the previous schematic)

The only thing that worries me now is possibility of clipping in the buffer.
How would one calculate that?

I read that high power (passive) humbuckers can produce up to 1 volt (peak).
On the Fetzer Valve website the calculator says I will get a gain of 5 (or 14dB).
So if I understand correctly, the buffer has to "withstand" a swing of +/- 5 volts?
Is this possible without the 1M/1M voltage divider?

Ok, I will go and test my FETs for current.
Does it have to be with battery, or can I use something else?
I have one small power supply I used for testing some guitar pedals (9v transformer, caps, LM7809 regulator).

Thanks again, man... you are very helpful :tu:
Faculty of sport you say? I'm at Faculty of civil engineering 

First, thank you very much for your answers! 

Ok, I've read some articles to learn what is this DC coupling you mentioned.
I think I get the basic idea... but since I learn as I go, more help will be needed 
(I study something completely different, so this is just my free-time "hobby")

So here goes (and correct me if I'm wrong):

Is this ok, or must the voltage divider (1M + 1M) of the buffer also be removed?
(there must be no clipping in the preamp!)


And now, the new set of questions:

1. First, you suggested direct coupled preamp. Why is that? Is the sound better?
(cost-wise there is no reason for leaving something out)

2. You said that less components are needed? Which ones would you omit and why? Did you mean those DC blocking capacitors and the pot?

3. If I understand correctly, now I can't just put a "volume" pot where it was. How would I lower the signal (voltage)? At the end of signal line? I would like to have some sort of "volume" control, because I'm affraid with some guitars (with active pickups) the output will be enough to clip my soundcard input. On the other side - with ten times lower output vintage pickups I may want to have all the gain I can.

4. And the final one: I only have these transistors available: j201, MPF102 and 2N5457. Which would make the best buffer? I found this page: "jFet bias calculator". I can measure both Vp and Idss of my transistors, but I do not understand the importance of Vrs or Vgs. 

Hello everyone, my first post here on SS guitar!  :tu:

I would like some opinions about a thing I'm trying to do.

I have a big tube amp (rack), but it's quite bulky.
Using it every time I have an idea and would like to record it... well, too unpractical
(takes too much time to plug everything, to warm up tubes, to set the microphone etc.)

I decided that for these quick ideas a "direct" recording would be the way to go (guitar into pc + some amp simulation software)
Since my soundcard does not have a high impedance input, I have to make one
(input on soundcard is unbalanced line-level)

So, here is the idea: I would like a high impedance input + some voltage gain. Since I like the sound of the "Fetzer Valve", this part is covered. I would also like a low impedance output, so I need a buffer.

I did these schematics in DIY Layout Creator, so you can see exactly what I'm doing:


Then, my idea was to simply join them together like this:


And now: the questions:
1. Would this work? (why / why not)
2. How can I improve this circuit? I've got Fetzer valve figured out, but I have no experience/understanding of buffers.
3. Do I really need the in/out capacitors in the buffer? How to they work (what do they do)?
4. Do I really need the 1M gate-source resistor in the buffer? Does it add any noise?
5. I like the 2N5457 in the fetzer valve, but would the MPF102 be better for the buffer?


Looking forward for your help... and thank you in advance :tu: